The present invention relates to a method of and an apparatus for measuring the cetane number of a diesel fuel.
Diesel fuels (otherwise known as gasoils) are designated with a cetane number (CN) which is an indication of the readiness of the diesel fuel to ignite when injected into the compressed air in the combustion chamber of a diesel engine. The higher the cetane number, the quicker is the ignition which follows the injection. Thus the cetane number is a characteristic of the self-ignition quality of the diesel fuel.
The cetane number is currently determined internationally by the procedure specified in ASTM D-613. This test is required to be manually performed on a sample of a finished blend of diesel fuel using a single cylinder four stroke diesel engine, having a variable compression ratio. The test requires a manual and critical adjustment of the quantity of injected fuel, as well as the injection timing and the compression ratio of the engine, in order to produce a constant ignition delay corresponding to a crank angle of 13xc2x0, (i.e. a delay of from 13xc2x0 before top dead centre to top dead centre). The test is performed on the finished blend sample, as well as on reference fuels having known cetane number, which are available from Phillips Petroleum of Bartlesville, Okla., USA. The cetane number of the finished blend sample is determined by a linear interpolation of the pre-chamber compression plug position found for the blended fuel, as compared to the corresponding positions for the reference fuels. This manual test lasts about one hour and is required to be performed on a sample of diesel fuel after blending. Thus the cetane number is only measured when the blending operation has been completed, thus preventing correction of the formulation of the diesel fuel blend on-line in the refinery.
Most importantly, the manual test also has poor precision. Although historically there has been much research worldwide to improve the inaccurate procedure of ASTM D-613, the repeatability of the test is as high as 0.9 cetane points and the reproducibility of the test is as high as 3.9 cetane points. No alternative test standard is accepted today, for example by the European Committee for Standardisation (CEN). Accordingly, the ASTM D-613 standard is likely to be in use for several years.
There are increasing commercial and technical requirements for cetane number determination to be accurately and reliably achievable. There is also an increasing demand for the cetane number to be determined on-line in the diesel fuel blending system. The cetane number has been included in the European diesel fuel specification EN590 specified by the European Committee for Standardisation. Since Oct. 1st, 1996 the cetane number has been required under EN590 to be at least 49 and the cetane number will need to be a minimum of 51 from Oct. 1st, 2000. It is likely that the diesel fuel standards will require increasingly higher cetane numbers in the future. The cetane number of the diesel fuel needs to be certified by the refiner. These increasingly higher requirements for the cetane number of diesel fuels will tend to force diesel fuel refiners to put commercially more valuable refinery streams and cetane improver additives into diesel fuel. This will make diesel fuel more expensive.
There is thus a need for refiners to be able to measure the cetane number of diesel fuels accurately and continuously on-line in the blending system so that a particular target threshold for the cetane value to meet the required standard can be achieved during the blending process without unnecessarily increasing the cetane number over the target threshold which constitutes a xe2x80x9cgive-awayxe2x80x9d of cetane number which is not commercially recoverable by the refiner. Currently, using the manual testing of finished blends, the blends can be found to be out of specification or can give-away cetane number, leading to significant loss of commercial value, and of time which is required for re-blending and re-testing of the fuel.
There have been prior proposals for the automatic on-line determination of cetane number in a CFR (Committee of Fuel Research) engine which is the engine specified in ASTM D-613.
Thus EP-A-0610118 discloses such a method which operates the CFR engine at constant compression ratio and compares the measured ignition delay of the blend sample with those of two reference fuels. Since this automatic process is an over-simplication of the process required by the ASTM D-613 standard, and in contrast to the standard does not have a constant ignition delay, this automatic process may not be used for product certification in accordance with that standard.
WO-A-97/39349 discloses an automated method which also requires the CFR engine to operate at constant compression ratio and compares the measured ignition delay of the blend sample with that of one reference fuel. The difference in ignition delay between the two fuels is employed to calculate a difference in cetane number, based on the calibration of the engine with two known reference fuels, once at the start of blending. Both the blend sample and the reference fuel have their own injection pump, which are each adjusted for flow and timing of the injection, the adjustment being at the start of blending. The cetane number determination is fed back to the adjustment of the components of the diesel fuel, and the mixture of cetane improver additives. Again, as this process is an over-simplication of the process specified in the ASTM D-613 standard, and does not have a constant ignition delay, it may not be used for product certification.
EP-A-0895080 also discloses a method of measuring cetane number of a diesel fuel in which the cetane number of a reference fuel is calculated using the difference between the average values of the ignition delay of the fuel to be measured and the reference fuel.
GB-A-2163878 discloses a process and device for adjusting the cetane rating of gasoil produced in a refinery. Again, during the measurement phase the compression ratio is maintained at a constant value.
U.S. Pat. No. 5,457,985 also discloses a process for measuring the cetane number of supply fuels for diesel engines which measures an autoignition delay of a diesel engine operating at constant speed with a constant volume compression ratio.
It has also been proposed to measure the cetane number of diesel fuels continuously by measuring the infra-red spectrum of the blended fuel and calculating the cetane number from algorithms based on a Fourier transform analysis. However, such calculation methods have a lower accuracy than the standard CFR methods discussed above.
There is a standard ASTM D-2885 for the on-line automatic measurement of the octane number of gasoline which is used by refineries during the gasoline blending process. The test procedure is based on an automated CFR octane engine, which compares the blend header gasoline with xe2x80x9cprototype fuelsxe2x80x9d. Such prototype fuels are themselves periodically compared with xe2x80x9cstandard fuelsxe2x80x9d, which have been separately tested (by xe2x80x9cround robinxe2x80x9d testing by a number of different refiners in the petroleum industry) in at least sixteen manual CFR engines. Such a process has been considered to have a better accuracy than manual determination, due to the use of the average octane number of the standard fuels which have been tested in xe2x80x9cround robinxe2x80x9d testing, and equally due to the large number of repetitions of the comparative measurement of the blend header octane number with that of the xe2x80x9cprototypexe2x80x9d fuels. Installations embodying this procedure are currently used all over the world. However, there is no such corresponding automatic cetane number determination process using the CFR engines meeting the parallel ASTM D-613 standard for the cetane number of diesel fuel.
In summary, there is a need in the art for a method of and apparatus for determining cetane number of diesel fuels which enables the on-line measurement of cetane number permitting certification of the cetane number of a batch of diesel fuel in accordance with accepted International standards, for example
There is also a need in the art for such a method and apparatus which enable cetane numbers to be determined not only automatically on-line but also with high accuracy, repeatability and reproducibility.
There is further a need in the art for such a method and apparatus which enable the cetane number of a diesel fuel to be controlled during blending to permit a target cetane value to be achieved reliably.
There is a yet further need in the art for a method and apparatus which can be readily integrated into a diesel fuel blending process and system to enable the cetane number of the blended diesel fuel to be controlled continuously.
The present invention aims at least partially to meet these needs.
Thus it is object of the present invention to provide a method of and an apparatus for determining the cetane number of a diesel fuel which are integratable into a diesel fuel blending system in a refinery for continuously determining the cetane number of the blend measured in accordance with an accepted standard.
It is a further object of the present invention to provide such a method and apparatus which can enable a continuously determined cetane number to be inputted into a control system for a diesel fuel blending system in a refinery to permit optimisation of the blend to achieve the objective of having the diesel fuel within the full specification with the least expensive blending additives and components available in the blend system.
Accordingly, the present invention provides a method of continually measuring the cetane number of a diesel fuel, the method comprising the steps of:
(a) providing a diesel engine having means for selectively supplying at least three diesel fuels to the engine, a first actuator for varying the injection timing of the engine, a second actuator for varying the fuel flow of the engine, and a third actuator for varying the compression ratio of the engine;
(b) providing a first supply of a diesel fuel of known cetane number, a second supply of a diesel fuel of different known cetane number and a third supply of a diesel fuel of unknown cetane number to be measured, the third supply being connected to a diesel fuel blending system;
(c) in a first cycle selectively and alternately supplying the first, second and third supplies to the diesel engine, and for each supply, controlling the first actuator to achieve an injection timing of a predetermined angle before top dead centre, controlling the second actuator to achieve a predetermined fuel flow and controlling the third actuator to achieve a predetermined diesel fuel ignition delay by varying the compression ratio of the engine;
(d) determining the cetane number of the third supply by linear interpolation of the pre-chamber plug positions, corresponding with the respective compression ratio values, for the three supplies; and
(e) periodically repeating steps (c) and (d) in further cycles to yield a series of cetane number values of the third supply.
The present invention further provides an apparatus for measuring the cetane number of a diesel fuel, the apparatus comprising a diesel engine having a fuel supply system for selectively supplying at least three diesel fuels to the engine, a first actuator for varying the injection timing of the engine, a second actuator for varying the fuel flow into the engine, and a third actuator for varying the compression ratio of the engine, the third actuator comprising a motor for displacing a pre-chamber plug of the engine.
The present invention yet further provides an apparatus for blending a diesel fuel, the apparatus comprising a diesel fuel blending system having at least one supply conduit for a plurality of diesel fuel components and additives, a diesel fuel outlet for the blend and a control system for controlling the composition of the components and additives in the blend, a diesel engine, a fuel supply system for the diesel engine, the fuel supply system being connected to the diesel fuel outlet and to two supplies of diesel fuels of known different cetane numbers, a first actuator for varying the injection timing of the engine, a second actuator for varying the fuel flow into the engine, a third actuator for varying the compression ratio of the engine, the third actuator comprising a motor for displacing a pre-chamber plug of the engine and a processing device for calculating the cetane number of the blended diesel fuel by comparison with the cetane numbers of the two supplies.
The present invention still further provides a method of blending a diesel fuel, the method comprising the steps of:
(a) providing a diesel engine having means for selectively supplying at least three diesel fuels to the engine, a first actuator for varying the injection timing of the engine, a second actuator for varying the fuel flow of the engine, and a third actuator for varying the compression ratio of the engine;
(b) providing a first supply of a diesel fuel of known cetane number, a second supply of a diesel fuel of different known cetane number and a third supply of a diesel fuel of unknown cetane number to be measured, the third supply being connected to a diesel fuel blending system to which is supplied a plurality of diesel fuel components and additives in amounts selected to achieve a target value for the cetane number of the blended fuel;
(c) in a first cycle selectively and alternately supplying the first, second and third supplies to the diesel engine, and for each supply controlling the first actuator to achieve an injection timing of a predetermined angle before top dead centre, controlling the second actuator to achieve a predetermined fuel flow, and controlling the third actuator to achieve a predetermined diesel fuel ignition delay by varying the compression ratio of the engine;
(d) determining the cetane number of the third supply by linear interpolation of the pre-chamber plug positions, corresponding with the respective compression ratio values, for the three supplies;
(e) periodically repeating steps (c) and (d) in further cycles to yield a series of cetane number values of the third supply; and
(f) calculating an average cetane number value of the blend.
The present invention can thus provide for the first time the automatic on-line measurement of the cetane number of a diesel fuel in accordance with the accepted standard (ASTM D-613), in particular during the blending process.